A spiral pipe has been widely used as a drainage in order to drain sewage, buried under the ground. A conventional spiral pipe has a spirally corrugated shape, and the structural strength thereof is good against load and impact. But, there is a disadvantage such that sewage can not be drained smoothly through the conventional spiral pipe due to the corrugations. In order to solve the shortcomings of the conventional spiral pipe, a spiral pipe with double walls has been developed.
As shown in FIG. 1A, a conventional spiral pipe is composed of an outer wall 1 which is prepared by winding spirally a steel plate 1a having corrugations 1b to thereby have a cylindrical configuration and an inner wall 2 having a cylindrical configuration, which contacts with the wave bottom of the corrugations of the outer wall 1. In a double-walled spiral pipe, the outer wall 1 having corrugations functions as a structure to resist against load and soil pressure, and the inner wall 2 having a plane surface, which is united with the wave bottom of the outer wall 1 along the inner surface of the corrugated spiral pipe. Therefore, the inner wall 2 functions to smoothly flow a sewage therethrough, and also a solid structure and an improved drainage capacity thereof can be expected.
As shown in FIG. 1B, in case that corrugations of the outer wall 1 are deformed due to load, soil pressure or heat, a pitch of corrugations of the spiral pipe can become wide or narrow, therefore a tensile force or compressive force are given to the inner wall 2. Relatively, the inner wall 2 having a poor resistance to a tensile force or compressive force can be easily damaged and also detached from the outer wall 1.
Also, since a corrugated surface 1b of the outer wall 1 is deformed depending upon heat, the inner wall 2 is repeatedly contracted and elongated. Therefore, a junction portion between the wave bottom of the corrugations of the outer wall 1 and the inner wall 2 can be broken.
As illustrated in above, the conventional double-walled spiral pipe has shortcomings that the outer wall can be easily deformed depending upon a sectional load and soil pressure, thereby the durability of the spiral pipe is reduced, and a sewage can not be drained smoothly through the inner pipe of the spiral pipe due to the deposit which is caused by the inflow of impurities or soil, and the inner wall can be damaged and detached from the outer wall depending upon heat.